With recent increase in integration of LSI, micromachining is being developed to form finer patterns utilizing the photolithography technology for exposure and transfer of integrated circuit onto silicon wafer. For that, wavelengths of light source become shorter and shorter and the silica glass, as an optical element of photolithography exposure apparatus, is demanded to have high transmittancy of light in the ultraviolet region and high uniformity of refractive index. In order to realize the high transmittancy of light in the ultraviolet region, it becomes necessary to lower an impurity concentration in silica glass and the flame hydrolysis process is used as a usual synthesizing method thereof.
This flame hydrolysis process includes heating a liquid material as a raw-material solution for glass at temperatures over the boiling point in a vaporization tank or the like to vaporize it, diluting this raw-material gas with a carrier gas such as oxygen, hydrogen and inert gases, and further blowing this raw-material gas, together with burning flame comprised of oxygen and hydrogen or combustible gas, from a burner for flame hydrolysis toward a target of heat-resisting material. Glass particles synthesized by a chemical reaction in this manner are fused into glass, thus synthesizing silica glass.
In the flame hydrolysis process the liquid material such as silicon tetrachloride is used as a raw material for glass, as described above, and it is used in the vaporized form. Accordingly, all metal impurities contained in the raw material for glass are trapped in the vaporization tank without provision of any special distillation device, which means that the raw material for glass is distilled. This permits high-purity raw-material gas to be supplied. In general, an evaporator such as the vaporizer is used as a device for supplying such high-purity raw-material gas.
This vaporizer is a gas control system for supplying the vaporized raw-material gas to a reaction system. Namely, the liquid material is vaporized under an ambient temperature over the boiling point thereof and the flow rate thereof is controlled stably and precisely by a mass flow controller. This vaporizer is basically comprised of a gas control section and a thermostatic bath.
Further, the glass raw material for optical fiber or silica glass is usually selected from liquid materials of silicon compounds such as silicon tetrachloride. This liquid material is supplied under pressure by nitrogen gas or the like from a vessel such as a bomb to the evaporator such as the vaporizer. More specifically, the supply under pressure of the liquid material to the evaporator is carried out by always keeping a constant pressure (approximately 1 to 2 kg/cm.sup.2) in the liquid material vessel through a regulator by nitrogen or inert gas (which may be either one of not only the inert gases in the periodic table, but also gases of low reactivity with the liquid material and which will be called pressure gas hereinafter). This pressurization causes the liquid material to be supplied to the evaporator and the gas is injected into the vessel by an amount of reduction of the liquid material.
A conventional liquid material supplying apparatus of this type is composed of a pressure gas line 10, a material supply line 20, and a vessel 50, as shown in FIG. 1. The pressure gas line 10 is composed of a gas supply pipe 11 through which the pressure gas to be injected into the vessel 50 is fed, a gas inlet pipe 51 fixed to the vessel 50, and a gas line connection pipe 13 having a gas line joint 15, being a joint to this gas inlet pipe 51, at the fore end. Further, the material supply line 20 is composed of a discharge pipe 53 fixed to the vessel 50, a material line connection pipe 23 having a material line joint 25, being a joint to this discharge pipe 53, at the end thereof, and a material supply pipe 27 connected to the vaporizer 91.
Further, the gas supply pipe 11 has an on-off valve as a gas feed valve 41 at the end thereof, and the gas line connection pipe 13 is connected thereto through this gas feed valve 41. This gas line connection pipe 13 branches to a by-pass pipe 29, the by-pass pipe 29 has an on-off valve as a line short valve 43 midway, and the other end of this by-pass pipe 29 is connected to the line connection pipe 23. This material line connection pipe 23 also has an on-off valve as a material acceptance valve 45 midway thereof.
Further, the material line connection pipe 23 is connected through a supply valve 47 to the material supply pipe 27, the by-pass pipe 29 is connected to between the material acceptance valve 45 and the material line joint 25, an exhaust pipe 31 is branched off therefrom between the material acceptance valve 45 and the supply valve 47, and the pipe 23 is connected through the exhaust pipe 31 having an on-off valve as an exhaust valve 49 to a harm eliminating column. Then the material supply pipe 27 is connected to the on-off valve being the supply valve 47 at one end and to the vaporizer 91 at the other end. Further, an on- off valve as a gas inlet valve 61 is provided midway on the gas inlet pipe 51 fixed to the vessel 50. In addition, an on-off valve as a discharge valve 63 is provided midway in the discharge pipe 53.
The above gas supply pipe 11, gas line connection pipe 13, material line connection pipe 23, material supply pipe 27, by-pass pipe 29, and valves etc. provided therein constitute a permanent line, while the above gas inlet pipe 51, discharge pipe 53, and valves provided therein constitute an attached line changed for another together with the vessel 50.
In the above conventional liquid material supplying apparatus, the pressure gas is fed through the gas supply pipe 11, gas line connection pipe 13, and gas inlet pipe 51 into the vessel 50, whereby the liquid material is supplied through the discharge pipe 53, the material line connection pipe 23, and the material supply pipe 27 to the vaporizer 91. When the liquid material remains little in the vessel 50, the gas is first fed through the by-pass pipe 29 and the material line connection pipe 23 into the exhaust pipe 31, thereby discharging the liquid material remaining in the material line connection pipe 23 to the exhaust pipe 31, and thereafter the vessel 50 is removed from the joints 15, 25 to be changed for a new vessel 50. Next, the pressure gas is fed into the vessel 50, thereby first causing the liquid material to push the pressure gas and air in the discharge pipe 53 and material line connection pipe 23 to the exhaust pipe 31, and thereafter supply of the liquid material to the vaporizer 91 is started again. Generally, such operation for discharging the remaining liquid material, the air in pipe, and so on is called a purge and the gas (pressure gas) used at that time is called purge gas.
There were, however, the following problems in supplying the liquid material to the vaporizer by the conventional method using the foregoing conventional apparatus.
First, the conventional method was unable to supply the raw-material gas continuously throughout a long period. Namely, there was a problem that supply of liquid material must be stopped temporarily on the occasion of necessity for replacement of vessel. In addition, upon this replacement of vessel it was necessary to undergo the above conventional purge, which required a long time for replacement of vessel. Therefore, supply of liquid material to the vaporizer was interrupted for a long time in the conventional case.
Because of this, the conventional method had a drawback that the replacement of vessel hindered stable supply of liquid material. Namely, when the liquid material having temporarily been stopped is supplied to the vaporizer after replacement of vessel and when the temperature of the liquid material supplied is low, a supply amount several ten or more times larger than a normal supply amount is supplied into the vaporization tank with a rush. There was thus a problem that the temperature of vaporizer decreased, so as to decrease an amount of gas generated. Then this resulted in changing an amount of gas supplied to the reaction system and in turn disabling stable production in the reaction system. This raised a problem of generating a heterogeneous characteristic called striae upon production of synthetic silica glass.
With the foregoing conventional method the liquid material remained as deposited on the discharge pipe and material line connection pipe, which made it difficult to purge the material from the material supply line completely and which required a very long time therefor. Then the incomplete purge raised the following problems.
Specifically, when a corrosive material such as silicon tetrachloride was used as the liquid material, a metal, even a corrosion resisting material such as SUS316L, touching such a liquid material was corroded by an acid such as hydrochloric acid made by reaction between water in the air and the liquid material. In addition, siloxane based compounds were also made by reaction with water in the air, and such reaction products and metal impurity particles became causes of blocking inside the mass flow controller connected to the vaporizer, and of flow-rate error. Further, there was also a problem that they became a cause of internal leak of valve.